Insulating glass units generally consist of two or more parallel panes of glass which are spaced apart from each other and which have the space between the panes sealed along the peripheries of the panes to enclose an air space between them. The most commonly used insulating glass units are double glazed windows. A double glazed window consists of two usually rectangular panes of glass which are placed in congruent relationship. Spacer bars are placed along the periphery of the space between the two panes. The spacer bars are long, hollow prisms having cross sections which are generally shaped in the form of isosceles trapezoids. The peripheries of the two panes and the spacer bars lying between them are sealed with a sealing composition so that the air space enclosed between the panes is sealed from contact with the outside atmosphere. The surfaces of the spacer bars facing the interior of the enclosed air space are perforated or slotted and the spacer bars themselves are filled with a solid adsorbent capable of taking up water vapor and organic materials which may be present in the enclosed air space when the unit is sealed with an organic sealant or which may enter the enclosed air space by diffusion from the sealant after sealing. Air enclosed in the space between the panes diffuses through the slots or perforations in the spacer bars and contacts the adsorbent in the interior of the spacer bars with the result that water vapor and any solvent or organic material getting into the enclosed air space from the sealing compound are adsorbed on the adsorbent employed. The result is that cooling of the interior air does not cause deposition of water vapor or organic material on the interior surfaces of the panes.
Insulating glass units of this design are frequently subjected to deflection of the glass panes due to pressure changes when the temperature of the outside air changes, adsorption or desorption of nitrogen or other gases on or from the adsorbent, and changes in atmospheric pressure. When the pressure of the air in the space enclosed between the panes becomes less than the exterior pressure, the panes are forced closer together. When the pressure in the space between the panes is greater than the exterior pressure the panes are forced apart. Since the peripheries of the panes are held in pretty much fixed position by the sealant deflection is observed to occur in the area of the glass lying inside the peripheries of the panes.
Deflection gives rise to several problems which must be faced by the manufacturer and/or the user of the insulated glass units. When appreciable deflection occurs the reflected images from the windows are distorted and present an undesirable cosmetic effect. This effect is not functionally serious but users of the insulating glass units object to the distorted reflections. Deflection which results in the movement of the two panes of glass closer together or farther apart when the exterior pressure is greater or less than the pressure of the enclosed air space between the panes places stress on the sealing compounds which lie along the periphery of the insulating glass unit and gradually weaken the seals so that leakage of the relatively moist exterior air into the enclosed space occurs with the result that the capacity of the adsorbent in the spacer bars is exhausted and condensation of moisture at low temperature begins to appear in the windows. Deflection which results when the panes are forced closer together decreases the insulating properties of the unit since these properties are a function of the width of the air space between the panes. If the panes are forced into contact with each other insulating properties are lost. Serious deflection can also cause cracking and even breakage of the windows particularly along the peripheries of the panes.
The deflection problem has been recognized and steps have been taken to reduce the amount of deflection experienced during transportation or use of the insulating glass units.
For example, it has been recognized that pressure problems arise when insulating glass units are shipped from a point of manufacture to a point of use and the altitudes between the two points are substantially different. In these situations it has been common practice to insert a small open tube, commonly known as a "breather tube", into the side of the spacer bar facing the exterior of the insulating glass unit. The breather tube permits flow of air between the interior of the insulating glass unit and the ambient atmosphere and thereby equilibrates the pressure. Typically, the breather tube is sealed immediately after the unit is transported to the altitude at which it is to be installed.
More recently it has been found that if the diameter of the breather tube is sufficiently small (of the order of 0.01 inch) and sufficiently long (generally of the order of at least one foot or more) entry of outside air into the insulating glass unit by simple diffusion is minimized and the insulating glass unit will exhibit sufficiently long life even if a breather tube of these dimensions is not sealed. It should be noted that breather tubes of this kind generally enter the side of the spacer bar facing the exterior of the insulating glass unit, and gas flow as air is "inhaled" into the air space enclosed between the panes of the unit is through the breather tube, through a small segment of the spacer bar and a small segment of the adsorbent contained in it with the flow of gas essentially perpendicular to the length of the spacer bar, then through the crack or slot or perforations in the spacer bar into the air space enclosed between the panes of the unit. During "exhaling" the gas flow is in the reverse direction.
Only recently it has been recognized that a serious cause of deflection in insulating glass units is the fact that the adsorbents with which the spacer bars have been filled adsorb nitrogen when the temperature in the interior of the space between the panes is low and desorb nitrogen when the temperature of the space between the panes is high. Deflection caused by nitrogen adsorption and desorption as temperature changes has been substantially eliminated by using adsorbents to fill the spacer bars which are incapable of adsorbing nitrogen but which do adsorb water vapor. This reduction of the nitrogen adsorption problem as relating to deflection is described in U.S. Pat. No. 4,144,196.